Did you read my post? HFC is quieter than an ACM mod in every way except HFC is much higher pitched and has no drone.

No CEL with active xpipe which I have.

Yes read your post but not sure what it has to do with what I am asking. I have the traditional 2 pipe oem mod, not the acm version. Also HFC = high flow cats correct? As I stated, I am not interested in messing with my primary cats, just looking for input from anyone who has removed secondary cats and what they thought of the results.

This...I'm in the same boat. I did the 2 pipe mod and LOVE the sound. Now I'm thinking I'd go for little power, and primary cat delete is the only cat you should be deleting for power because secondary flows well already. Leaving the resonator in place to. Too much sound increase and having to get a tune for CEL is what's holding me back. Not sure if it's all worth it honestly. These cars are plenty fast, I just want the S65 to breathe her best. Anyway, you should either do the primary or don't bother IMO.

I have a loud car already, the M3 is my daily car so I don't want it too loud, just want to brighten up the tone, and definitely no drone. I already bought the Evolve tune, haven't decided weather to get the HFC or test pipe yet. HFC are expensive but it should sound just right. Test pipe might be smelly and might be a bit too loud for me. Does anyone know how much louder is the test pipe comparing to HFC?

Sorry I didn't explain fully. The second cat flows very well and the big restriction is the primary. My point was about sound and HP. Too much sound in my opinion with little HP increase. If you were going to mess with it, go with primary. It's your car though.